In conventional cellular systems a mobile terminal is connected to a single cell and when that terminal moves from the serving area of one cell to the serving area of another cell, typically a handover is initiated. The full picture of all cells in the area surrounding the terminal is only available at the terminal itself. However, a general paradigm of a well-organized cellular network is that the network makes the mobility decisions, and not the terminal.
One solution to get all the information at the user equipment (UE) about its surrounding cells would for the terminal to permanently send measurement reports to the network, but this would require too much signalling overhead. Instead, current E-UTRAN LTE (evolved UMTS Terrestrial Radio Access Network, Long Term Evolution, also known as 4G) specifications allow the network to configure triggers for the terminal. If such a network-configured trigger expires in the terminal the terminal will in response send a measurement report. The idea is that the network configures the triggers such that a handover is initiated when such a measurement report is received. This minimizes the signalling overhead by limiting the number of measurement reports that are sent.
For intra-frequency handovers in LTE, the most prominent trigger for a measurement report is the A3 trigger which in simplified terms is defined as follows:Mn+Ocn>Ms+Off.
It expires if the measurement Mn of a neighbor n is offset (the value of Off) better than the measurement Ms of a serving cell. The measurements could be given as signal strength (such as reference signal received power/RSRP) or as a signal quality (such as reference signal received quality/RSRQ). The offset Off introduces a kind of hysteresis to the handover decision to avoid the well-known ‘ping-pong’ effect.
Ocn is another offset (also called “cell individual offset”) which, in contrast to Off is specific to an individual neighbor cell. It can be used to fine-adjust the handovers individually towards different neighbor cells due to mobility robustness reasons (e.g., make the neighbor more attractive if it is entered through a high-speed street), or due to load balancing reasons (e.g., make the neighbor cell more attractive if it is experiencing low traffic loading).
A brief overview on seven of the LTE measurement report triggers may be seen at http://www.rfwireless-world.com/Terminology/LTE-UE-Event-Measurement-Reporting.html (last visited Jun. 23, 2015). Further detail on known measurement report triggers may be seen at international patent publication WO 2014/021763, as well as technical specifications 25.331 (v12.5.0) at section 8.4 “Measurement procedures”; 36.300 (v12.5.0) at sections 10.1.3 and 10.2.3 each entitled “Measurements”; and 36.331 (v12.5.0) at section 5.5.4 “Measurement report triggering”.
Another option is to dispense with the terminals' measurement reports and instead have the network perform measurements itself on the terminals' respective uplink signalling. This option makes several assumptions, namely that the terminals are permanently transmitting (such that the network can measure at all), that the network will know with a reasonably high degree of accuracy the terminals' respective transmit powers, and that the cells will be able to exchange all these various uplink measurements with one another in a timely manner. But the circumstances under which all those assumptions would hold true collide with several other requirements such as the terminal's energy consumption in view of its limited portable power supply. The inventors see this option as a supplement to support or improve terminal measurements but not suitable to replace them in a practical system.
In E-UTRAN LTE, the Cooperative Multi-Point (CoMP) transmission scheme allows a given user equipment (UE) to be served by multiple cells simultaneously, but still only one is handling the control plane of the UE and is generally referred to as the primary cell or PCell, while the other serving cells are secondary cells or SCells. The connection of the terminal purely depends on the PCell which is changed by a conventional handover. If the control plane on the PCell has radio problems, the other SCells cannot serve as a fallback.
In 3G soft handover, a UE is configured with an active set of cells that transmit the same content to the UE on independent links which are combined at the receiver side, i.e., control information is sent from each cell. Similar to 3G soft handover, it is anticipated that in future cellular systems such as 5G a given UE may have multiple serving cells in the active set handling both the user and control planes. Otherwise, the connection would rely only on a single cell, and there would be no control-channel robustness benefit. Whereas in LTE the UE's active set refers to its PCell and any of the SCells, in the description below the term ‘active set’ refers to all the UE's simultaneous serving cells handling both user and control planes of the UE (and for simplicity of explanation it is assumed below that all these active cells are on the same frequency layer, or ‘intra-frequency’ cells). In the overview of FIG. 1 assume the UE has 3 serving cells as shown and is moving towards a new cell that is not (yet) in its active set. The parameters of trigger events defined for instance in 3G for adding, removing or replacing a cell in the active set are fixed and they are not tailored for each UE leading to sub-optimal performance in terms of throughput and number of active set updates. The examples below address how to solve this problem.